Electrochromic device having a self-cleaning hydrophilic coating

ABSTRACT

An electrochromic device is disclosed having a self-cleaning, hydrophilic optical coating. The electrochromic device preferably forms an external rearview mirror for a vehicle The coating preferably includes alternating layers of a photocatalytic material having a high index of refraction and a hydrophilic material having a low refractive index More specifically, the coating includes a first layer having a high refractive index, a second layer having a low refractive index, a third layer of titanium dioxide, and a fourth layer of silicon dioxide provided as an outermost layer. The disclosed optical coating exhibits a reflectance at the front surface of the reflective element that is less than about 20 percent, and has sufficient hydrophilic properties such that water droplets on a front surface of the optical coating exhibit a contact angle of less than about 20°.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C §119(e) on U.S.Provisional Patent Application No 60/141,080, entitled “ANELECTROCHROMIC DEVICE HAVING A SELF-CLEANING HYDROPHILIC COATING,” andfiled on Jun. 25, 1999, the entire disclosure of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to electrochromicdevices, and more specifically relates to electrochromic rearviewmirrors of a vehicle.

[0003] To enable water droplets and mist to be readily removed from thewindows of a vehicle, the windows are typically coated with ahydrophobic material that causes the water droplets to bead up on theouter surface of the window. These water beads are then either sweptaway by windshield wipers or are blown off the window as the vehiclemoves It is equally desirable to clear external rearview mirrors ofwater. However, if a hydrophobic coating is applied to the externalmirrors, the water beads formed on their surfaces cannot be effectivelyblown off since such mirrors are relatively shielded from direct airflowresulting from vehicle movement Thus, water droplets or beads that areallowed to form on the surface of the mirrors remain on the mirror untilthey evaporate or grow in size until they fall from their own weight.These water droplets act as small lenses and distort the image reflectedto the driver. Further, when the water droplets evaporate, water spotsare left on the mirror, which are nearly as distracting as the waterdroplets that left the spots In fog or high humidity, mist forms on thesurfaces of the external mirrors. Such a mist call be so dense that iteffectively renders the mirrors virtually unusable

[0004] In an attempt to overcome the above-noted problems, mirrormanufacturers have provided a hydrophilic coating on the outer surfaceof the external mirrors. See U.S. Pat. No. 5,594,585. One suchhydrophilic coating includes a single layer of silicon dioxide (SiO₂)The SiO₂ layer is relatively porous. Water on the mirror Is absorbeduniformly across the surface of the mirror into the pores of the SiO₂layer and subsequently evaporates leaving no water spots. One problemwith such single layer coatings of SiO₂ is that oil, grease, and othercontaminants can also fill the pores of the SiO₂ layer Many suchcontaminants, particularly hydrocarbons like oil and grease, do notreadily evaporate and hence clog the pores of the SiO₂ layer. When thepores of the SiO₂ layer become clogged with car wax, oil, and grease,the mirror surface becomes hydrophobic and hence the water on the mirrortends to bead leading to the problems noted above.

[0005] A solution to the above problem pertaining to hydrophilic layersis to form the coating of a relatively thick layer (e.g., about1000-3000 Å or more) of titanium dioxide (TiO₂) See European PatentApplication Publication No. EPO 816 466 A1. This coating exhibitsphotocatalytic properties when exposed to ultraviolet (UV) radiation.More specifically, the coating absorbs UV photons and, in the presenceof water, generates highly reactive hydroxyl radicals that tend tooxidize organic materials that have collected in its pores or on itssurface Consequently, hydrocarbons, such as oil and grease, that havecollected on the mirror are converted to carbon dioxide (CO₂) and henceis eventually removed from the mirror whenever UV radiation impingesupon the mirror surface. This particular coating is thus a self-cleaninghydrophilic coating.

[0006] One measure of the hydrophilicity of a particular coating is tomeasure the contact angle that the sides of a water drop form with thesurface of the coating An acceptable level of hydrophilicity as presentin a mirror when the contact angle is less than about 30°, and morepreferably, the hydrophilicity is less than about 20° The aboveself-cleaning hydrophilic coating exhibits contact angles that decreasewhen exposed to UV radiation as a result of the self-cleaning action andthe hydrophilic effect of the coating The hydrophilic effect of thiscoating, however, tends to reverse over time when the mirror is notexposed to UV radiation.

[0007] The above self-cleaning hydrophilic coating can be improved byproviding a film of about 150 to 1000 Å of SiO₂ on top of the relativelythick TiO₂ layer. See U.S. Pat. No. 5,854,708 This seems to enhance theself-cleaning nature of the TiO₂ layer by reducing the dosage of UVradiation required and by maintaining the hydrophilic effect of themirror over a longer period of time after the mirror is no longerexposed to UV radiation.

[0008] While the above hydrophilic coatings work well on conventionalrearview mirrors having a chrome or silver layer on the rear surface ofa glass substrate, they have not been considered for use onelectrochromic mirrors for several reasons. A first reason is that manyof the above-noted hydrophilic coatings introduce colored double imagesand increase the low-end reflectivity of the electrochromic mirror. Forexample, commercially available, outside electrochromic mirrors existthat have a low-end reflectivity of about 10 percent and a high-endreflectivity of about 50 to 65 percent. By providing a hydrophiliccoating including a material such as TiO₂, which has a high index ofrefraction, on a glass surface of the mirror, a significant amount ofthe incident light is reflected at the glass/TiO₂ layer interfaceregardless of the variable reflectivity level of the mirror. Thus, thelow-end reflectivity would be increased accordingly. Such a higherlow-end reflectivity obviously significantly reduces the range ofvariable reflectance the mirror exhibits and thus reduces theeffectiveness of the mirror in reducing annoying glare from theheadlights of rearward vehicles.

[0009] Another reason that the prior hydrophilic coatings have not beenconsidered for use on electrochromic elements is that they impartsignificant coloration problems. Coatings such as those having a 1000 Ålayer of TiO₂ covered with a 150 Å layer of SiO₂, exhibit a very purplehue When used in a conventional mirror having chrome or silver appliedto the rear surface of a glass element, such coloration is effectivelyreduced by the highly reflective chrome or silver layer, since the colorneutral reflections from the highly reflective layer overwhelm thecoloration of the lower reflectivity, hydrophilic coating layer.However, if used on an electrochromic element, such a hydrophiliccoating would impart a very objectionable coloration, which is madeworse by other components in the electrochromic element that can alsointroduce color.

[0010] Due to the problems associated with providing a hydrophiliccoating made of TiO₂ on an electrochromic mirror, manufacturers of suchmirrors have opted to not use such hydrophilic coatings. As a result,electrochromic mirrors suffer from the above-noted adverse consequencescaused by water drops and mist.

SUMMARY OF THE INVENTION

[0011] Accordingly, it is an aspect of the present invention to solvethe above problems by providing a hydrophilic coating suitable for useon an electrochromic device, particularly for an electrochromic mirror.To achieve these and other aspects and advantages, a rearview mirroraccording to the present invention comprises an electrochromic mirrorelement having a reflectivity that may be varied in response to anapplied voltage so as to exhibit at least a high reflectance state andlow reflectance state, and a hydrophilic optical coating applied to afront surface of the electrochromic mirror element. The rear-view mirrorpreferably exhibits a spectral reflectance of less than 20 percent insaid low reflectance state, and also preferably exhibits a C* value lessthan about 20 in both said high and low reflectance states so as toexhibit substantial color neutrality

[0012] These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In the drawings

[0014]FIG. 1 is a front perspective view of an external electrochromicrearview mirror assembly constructed in accordance with the presentinvention; and

[0015]FIG. 2 is a cross section of the external electrochromic rearviewmirror assembly shown in FIG. 1 along line II-II.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016]FIG. 1 shows an external electrochromic rearview mirror assembly10 constructed in accordance with the present invention. As shown,mirror assembly 10 generally includes a housing 15 and a mirror 20movably mounted in housing 15. Housing 15 may have any conventionalstructure suitably adapted for mounting assembly 10 to the exterior of avehicle.

[0017]FIG. 2 shows an exemplary construction of mirror 20. As broadlydescribed herein, mirror 20 includes a reflective element 100 having areflectivity that may be varied in response to an applied voltage and anoptical coating 130 applied to a front surface 112 a of reflectiveelement 100. Reflective element 100 preferably includes a first (orfront) element 112 and a second (or rear) element 114 sealably bonded inspaced-apart relation to define a chamber Front element 112 has a frontsurface 112 a and a rear surface 112 h, and rear element 114 has a frontsurface 114 a and a rear surface 114 b. For purposes of furtherreference, front surface 112 a of front element 112 shall be referred toas the first surface, rear surface 112 b of front element 112 shall bereferred to as the second surface, front surface 114 a of rear element114 shall be referred to as the third surface, and rear surface 114 b ofrear element 114 shall be referred to as the fourth surface ofreflective element 100 Preferably, both elements 112 and 114 aretransparent and are scalably bonded by means of a seal member 116

[0018] Reflective element 100 also includes a transparent firstelectrode 118 carried on one of second surface 112 b and third surface114 a, and a second electrode 120 carried on one of second surface 112 band third surface 114 a. Second electrode 120 may be reflective ortransflective, or a separate reflector 122 may be provided on fourthsurface 114 b of mirror 100 in which case electrode 120 would betransparent. Preferably, however, second electrode 120 is reflective ortransflective and the layer referenced by numeral 122 is an opaque layeror omitted entirely Reflective element 100 also includes anelectrochromic medium 124 contained in the chamber in electrical contactwith first and second electrodes 118 and 120.

[0019] Electrochromic medium 124 includes electrochromic anodic andcathodic materials that can be grouped into the following categories:

[0020] (i) Single layer—the electrochromic medium is a single layer ofmaterial which may include small nonhomogeneous regions and includessolution-phase devices where a material is contained in solution in theionically conducting electrolyte and remains in solution in theelectrolyte when electrochemically oxidized or reduced. Solution-phaseelectroactive materials may be contained In the Continuous Solutionphase of a cross-linked polymer-matrix in accordance with the teachingsof U.S. patent application Ser. No. 08/616,967, entitled “IMPROVEDELECTROCHROMIC LAYER AND DEVICES COMPRISING SAME” or internationalPatent Application No. PCT/US98/05570 entitled “ELECTROCHROMIC POLYMERICSOLID FILMS, MANUFACTURING ELECTROCHROMIC DEVICES USING SUCH SOLIDFILMS, AND PROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES.”

[0021] At least three electroactive materials, at least two of which areelectrochromic, can be combined to give a pre-selected color asdescribed in U.S. patent application Ser. No. 08/832,596 entitled “ANIMPROVED ELECTROCHROMIC MEDIUM CAPABLE OF PRODUCING A PRE-SELECTEDCOLOR.”

[0022] The anodic and cathodic materials can be combined or linked by abridging unit as described in International Application No.PCT/WO97/EP498 entitled “ELECTROCHROMIC SYSTEM.” It is also possible tolink anodic materials or cathodic materials by similar methods. Theconcepts described in these applications can further be combined toyield a variety of electrochromic materials that are linked.

[0023] Additionally, a single layer medium includes the medium where theanodic and cathodic materials can be incorporated into the polymermatrix as described in International Application No. PCT/WO98/EP3862entitled “ELECTROCHROMIC POLYMER SYSTEM” or International PatentApplication No. PCT/US98/05570 entitled “ELECTROCHROMIC POLYMERIC SOLIDFILMS, MANUFACTURING ELECTROCHROMIC DEVICES USING SUCH SOLID FILMS, ANDPROCESSES FOR MAKING SUCH SOLID FILMS AND DEVICES.”

[0024] Also included is a medium where one or more materials in themedium undergoes a change in phase during the operation of the device,or example, a deposition system where a material contained in solutionin the ionically conducting electrolyte, which forms a layer or partiallayer on the electronically conducting electrode when electrochemicallyoxidized or reduced.

[0025] (ii) Multilayer—the medium is made up in layers and includes atleast one material attached directly to an electronically conductingelectrode or confined in close proximity thereto, which remains attachedor confined when electrochemically oxidized or reduced. Examples of thistype of electrochromic medium are the metal oxide films, such astungsten oxide, iridium oxide, nickel oxide, and vanadium oxide. Amedium, which contains one or more organic electrochromic layers, suchas polythiophene, polyaniline, or polypyrrole attached to the electrode,would also be considered a multilayer medium.

[0026] In addition, the electrochromic medium may also contain othermaterials, such as light absorbers, light stabilizers, thermalstabilizers, antioxidants, thickeners, or viscosity modifiers

[0027] Because reflective element 100 may have essentially anystructure, the details of such structures are not further described.Examples of preferred electrochromic mirror constructions are disclosedin U.S. Pat. No. 4,902,108, entitled “SINGLE-COMPARTMENT, SELF-ERASING,SOLUTION-PHASE ELECTROCHROMIC DEVICES SOLUTIONS FOR USE THEREIN, ANDUSES THEREOF,” issued Feb. 20, 1990, to H.J. Byker; Canadian Patent No.1,300,945, entitled “AUTOMATIC REARVIEW MIRROR SYSTEM FOR AUTOMOTIVEVEHICLES,” issued May 19, 1992, to J. H. Bechtel et al.: U.S. Pat. No.5,128,799, entitled “VARIABLE REFLECTANCE MOTOR VEHICLE MIRROR,” issuedJul. 7, 1992, to H.J. Byker; U.S. Pat. No. 5,202,787, entitled“ELECTRO-OPTIC DEVICE,” issued Apr. 13, 1993 to H J Byker et al. U.S.Pat. No. 5,204,778, entitled “CONTROL SYSTEM FOR AUTOMATIC REARVIEWMIRRORS,” issued Apr. 20, 1993, to J. H. Bechtel, U.S. Pat. No.5,278,693, entitled “TINTED SOLUTION-PHASE ELECTROCHROMIC MIRRORS,”issued Jan. 11, 1994, to D A Theiste et al.; U.S. Pat. No. 5,280,380,entitled “UV STABILIZED COMPOSITIONS AND METHODS,” issued Jan. 18, 1994,to H J. Byker; U.S. Pat. No. 5,282,077, entitled “VARIABLE REFLECTANCEMIRROR,” issued Jan. 25, 1994, to H. J. Byker, U.S. Pat. No. 5,294,376,entitled “BIPYRIDINIUM SALT SOLUTIONS,” issued Mar. 15, 1994, to H.J.Byker; U.S. Pat. No. 5,336,448, entitled “ELECTROCHROMIC DEVICES WITHBIPYRIDINIUM SALT SOLUTIONS,” issued Aug. 9, 1994, to H.J. Byker; U.S.Pat. No. 5,434,407, entitled “AUTOMATIC REARVIEW MIRROR INCORPORATINGLIGHT PIPE,” issued Jan. 18, 1995, to F. T. Bauer et al.; U.S. Pat. No.5,448,397, entitled “OUTSIDE AUTOMATIC REARVIEW MIRROR FOR AUTOMOTIVEVEHICLES,” issued Sep. 5, 1995, to W. L. Tonar; U.S. Pat. No. 5,451,822,entitled “ELECTRONIC CONTROL SYSTEM,” issued Sep. 19, 1995, to J. H.Bechtel et al.; U.S. Pat. No. 5,818,625, entitled “ELECTROCHROMICREARVIEW MIRROR INCORPORATING A THIRD SURFACE METAL REFLECTOR,” byJeffrey A. Forgette et al.; and U.S. patent application Ser. No.09/158,423, entitled “IMPROVED SEAL FOR ELECTROCHROMIC DEVICES,” filedon Sep. 21, 1998. Each of these patents and the patent application arecommonly assigned with the present invention and the disclosures ofeach, including the references contained therein, are herebyincorporated herein in their entirety by reference.

[0028] If the mirror assembly includes a signal light, display, or otherindicia behind the reflective electrode or reflective layer ofreflective element 100, reflective element 100 is preferably constructedas disclosed in commonly assigned U.S. patent application Ser. No.09/311,955, entitled “ELECTROCHROMIC REARVIEW MIRROR INCORPORATING ATHIRD SURFACE METAL REFLECTOR AND A DISPLAY/SIGNAL LIGHT,” filer on May14, 1999, by W. L. Tonar et al., the disclosure of which is incorporatedherein by reference. If reflective element 100 is convex or aspheric, asis common for passenger-side external rearview mirrors as well asexternal driver-side rearview mirrors of cars in Japan and Europe,reflective element 100 may be made using thinner elements 112 and 114while using a polymer matrix in the chamber formed therebetween as isdisclosed in commonly assigned U.S. patent application Ser. No.08/834,783 entitled “AN ELECTROCHROMIC MIRROR WITH TWO THIN GLASSELEMENTS AND A GELLED ELECTROCHROMIC MEDIUM,” filed on Apr. 2, 1997. Theentire disclosure, including the references contained therein, of thisU.S. patent application is incorporated herein by reference. Theaddition of the combined reflector/electrode 120 onto third surface 114a of reflective element 100 further helps remove any residual doubleimaging resulting from the two glass elements being out of parallel Theelectrochromic element of the present invention is preferably colorneutral. In a color neutral electrochromic element, the element darkensto a gray color, which is more ascetically pleasing than any other colorwhen used in an electrochromic mirror. U.S. patent application Ser. No.08/832,596, entitled “AN IMPROVED ELECTROCHROMIC MEDIUM CAPABLE OFPRODUCING A PRE-SELECTED COLOR” discloses electrochromic media that areperceived to be gray throughout their normal range of operation Theentire disclosure of this application is hereby incorporated herein byreference. U.S. patent application Ser. No., 09/311,955 entitled“ELECTROCHROMIC REARVIEW MIRROR INCORPORATING A THIRD SURFACE METALREFLECTOR AND A DISPLAY/SIGNAL LIGHT” discloses additionalelectrochromic mirrors that exhibit substantial color neutrality whileenabling displays to be positioned behind the reflective surface of theelectrochromic mirror The entire disclosure of this application ishereby incorporated herein by reference

[0029] In addition to reflective element 100, mirror 20 further includesan optical coating 130 Optical coating 130 is a self-cleaninghydrophilic optical coating. Optical coating 130 preferably exhibits areflectance at first surface 112 a of reflective element 100 that isless than about 20 percent. If the reflectance at first surface 112 a isgreater than about 20 percent, noticeable double-imaging results, andthe range of variable reflectance of reflective element 100 issignificantly reduced. The electrochromic mirror as a unit should have areflectance of less than about 20 percent in its lowest reflectancestate, and more preferably should have a reflectance of less than about10 percent.

[0030] Optical coating 130 also is preferably sufficiently hydrophilicsuch that water droplets on a front surface of coating 130 exhibit acontact angle of less than about 30°, more preferably less than about200, and most preferably less than about 10°. If the contact angle isgreater than about 30°, the coating 130 exhibits insufficienthydrophilic properties to prevent distracting water beads from formingOptical coating 130 should also exhibit self cleaning properties wherebythe hydrophilic properties may be restored following exposure to UVradiation.

[0031] In one embodiment, optical coating 130 includes at least fourlayers of alternating high and low refractive index. Specifically, asshown in FIG. 2, optical coating 130 includes, in sequence, a firstlayer 132 having a high refractive index, a second layer 134 having alow refractive index, a third layer 136 having a high refractive index,and a fourth layer 138 having a low refractive index. Preferably, thirdlayer 136 is made of a photocatalytic material, and fourth layer 138 ismade of a material that will enhance the hydrophilic properties of thephotocatalytic layer 136 by generating hydroxyl groups on its surfaceSuitable hydrophilic enhancement materials include SiO₂ and Al₂O₃, withSiO₂ being most preferred. Suitable photocatalytic materials includeTiO₂, ZnO, ZnO₂, SnO₂, ZnS, CdS, CdSe, Nb₂O₅, KTaNbO₃, KTaO₃, SrTiO₃,WO₃, Bi2O₃, Fe2O₃, and GaP, with TiO₂ being most preferred By making theoutermost layers TiO₂ and SiO₂, coating 130 exhibits good self-cleaninghydrophilic properties similar to those obtained by the prior arthydrophilic coatings applied to conventional mirrors having a reflectorprovided on the rear surface of a single front glass element.Preferably, the thickness of the SiO outer layer is less than about 800Å. If the SiO₂ outer layer is too thick (e.g., more than about 1000 Å),the underlying photocatalytic layer will not be able to “clean” the SiO₂hydrophilic outer layer, at least not within a short time period. Thetwo additional layers (layers 132 and 134) are provided to reduce theundesirable reflectance levels at the front surface of reflectiveelement 100. Preferably, layer 132 is made of a photocatalytic materialand second layer 134 is made of a hydrophilic enhancement material so asto contribute to the hydrophilic and photocatalytic properties of thecoating. Thus, layer 132 may be made of any one of the photocatalyticmaterials described above or mixtures thereof, and layer 134 may be madeof any of the hydrophilic enhancement materials described above ormixtures thereof. Preferably layer 132 is made of TiO₂, and layer 134 ismade of SiO₂.

[0032] An alternative technique to using a high index layer and lowindex layer between the glass and the layer that is primarily comprisedof photocatalytic metal oxide (i e, layer 136) to obtain all of thedesired properties while maintaining a minimum top layer thickness ofprimarily silica is to use a layer, or layers, of intermediate index.This layer(s) could be a single material such as tin oxide or a mixtureof materials such as a blend of titania and silica. Among the materialsthat have been modeled as potentially useful are blends of titania andsilica, which can be obtained through sol-gel deposition as well asother means, and tin oxide One can use a graded index between the glassand layer primarily composed of photocatalytic material as well.

[0033] Additionally, one can obtain roughly the same color andreflectance properties with a thinner top layer or possibly no top layercontaining primarily silica if the index of the photocatalytic layer islowered somewhat by blending materials, as would be the case, forexample, for a titania and silica mixture deposited by sol-gel. Thelower index of the titania and silica blend layer imparts lessreflectivity, requires less compensation optically, and therefore allowsfor a thinner top layer. This thinner top layer should allow for more ofthe photocatalytic effect to reach surface contaminants.

[0034] The index of refraction of a titania film obtained from a givencoating system can vary substantially with the choice of coatingconditions and could be chosen to give the lowest index possible whilemaintaining sufficient amounts of anatase or rutile form in the film anddemonstrating adequate abrasion resistance and physical durability. Thelower index obtained in this fashion would yield similar advantages tolowering the index by mixing titania with a lower index material. RonWilley, in his book “Practical Design and Production of Optical ThinFilms,” Marcel Dekker, 1996, cites an experiment where temperature ofthe substrate, partial pressure of oxygen, and speed of deposition varythe index of refraction of the titania deposited from about n=2 1 ton=2.4

[0035] Materials used for transparent second surface conductors aretypically materials whose index of refraction is about 1.9 or greaterand have then color minimized by using half wave thickness multiples orby using the thinnest layer possible for the application or by the useof one of several “non-iridescent glass structures.” Thesenon-iridescent structures will typically use either a high and low indexlayer under the high index conductive coating (see, for example, U.S.Pat. No. 4,377,613 and U.S. Pat. No. 4,419,386 by Roy Gordon), or anintermediate index layer (see U.S. Pat. No. 4,308,316 by Roy Gordon) orgraded index layer (see U.S. Pat. No. 4,440,822 by Roy Gordon).

[0036] Fluorine doped tin oxide conductors using a non-iridescentstructure are commercially available from Libbey-Owens-Ford and are usedas the second surface transparent conductors in most inside automotiveelectrochromic mirrors produced at the present time. The dark statecolor of devices using this second surface coating stack is superior tothat of elements using optical half wave thickness indium tin oxide(ITO) when it is used as a second surface conductive coating. Drawbacksof this non-iridescent coating are mentioned elsewhere in this document.Hydrophilic and photocatalytic coating stacks with less than about 800 Åsilica top layer, such as 1000 Å titania 500 Å silica, would stillimpart unacceptable color and/or reflectivity when used as a firstsurface coating stack in conjunction with this non-iridescent secondsurface conductor and other non-iridescent second surface structures,per the previous paragraph, that are not designed to compensate for thecolor of hydrophilic coating stacks on the opposing surface. Techniqueswould still need to be applied per the present embodiment at the firstsurface to reduce C* of the system in the dark state if these coatingswere used on the second surface

[0037] ITO layers typically used are either very thin (approximately200-250 Å) which minimizes the optical effect of the material by makingit as thin as possible while maintaining sheet resistances adequate formany display devices, or multiples of half wave optical thickness (about1400 Å), which minimizes the overall reflectivity of the coating. Ineither case, the addition of photocatalytic hydrophilic coating stackson opposing surfaces per the previous paragraph would createunacceptable color and/or reflectivity in conjunction with the use ofthese layer thicknesses of ITO used as the second surface conductor.Again, techniques would need to be applied per the present embodiment atthe first surface to reduce the C* of the system in the dark state.

[0038] In somewhat analogous fashion, for modification of the firstsurface-coating stack to optimize the color and reflectivity of thesystem containing both first and second surface coatings, one can modifythe second surface-coating stack to optimize the color of the system.One would do this by essentially creating a compensating color at thesecond surface in order to make reflectance of the system more uniformacross the visible spectrum, while still maintaining relatively lowoverall reflectance For example, the 1000 Å titania 500 Å silica stackdiscussed in several places within this document has a reddish-purplecolor due to having somewhat higher reflectance in both the violet andred portions of the spectrum than it has in the green. A second surfacecoating with green color, such as {fraction (3/4)} wave opticalthickness ITO, will result in a lower C* value for the dark state systemthan a system with a more standard thickness of ITO of half wave opticalthickness, which is not green in color. Additionally, one can modifythicknesses of layers or choose materials with somewhat differentindices in the non-iridescent structures mentioned in order to create acompensating color second surface as well.

[0039] Another method of color compensating the first surface is throughpre selecting the color of the electrochromic medium in the dark statein accordance with the teachings of commonly assigned U.S. patentapplication Ser. No 08/832,596, entitled “AN IMPROVED ELECTROCHROMICMEDIUM CAPABLE OF PRODUCING A PRE-SELECTED COLOR.” Again, by using firstsurface coatings of 1000 Å titania followed by 500 Å silica as anexample, the following modification would assist in lowering the C*value of an electrochromic mirror when activated. If, in that case, thecolor of the electrochromic medium was selected so that it was lessabsorbing in the green region when activated, the higher reflection ofgreen wavelengths of light from the third or fourth surface reflector ofthe element would help balance the reflection of the unit in the darkstate.

[0040] Combinations of the aforementioned concepts for the first, secondsurface, and electrochromic medium are also potentially advantageous forthe design

[0041] It is known that the optical properties for a deposited film varydepending on deposition conditions that include partial pressure ofoxygen gas, temperature of the substrate speed of deposition, and thelike. In particular, the index of refraction for a particular set ofparameters on a particular system will affect the optimum layerthicknesses for obtaining the optical properties being discussed.

[0042] The discussions regarding the photocatalytic and hydrophilicproperties of titania and like photocatalytic materials and silica andlike hydrophilic materials are generally applicable to layers of mixedmaterials as long as the mixtures retain the basic properties ofphotocatalytic activity and/or hydrophilicity. Abrasion resistance isalso a major consideration in the outermost layer. UP 0816466A1describes an abrasion resistant, photocatalytic, hydrophilic layer ofsilica blended titania, as well as a layer of tin oxide blended titaniawith similar properties. U.S. Pat. No. 5,755,867 describesphotocatalytic blends of silica and titania obtained through use ofthese mixtures. These coatings would likely require modifications tochange their optical properties suitable for use on an electrochromicdevice The potential advantages of these optical property modificationsto this invention are discussed further below.

[0043] In some variations of this invention, it may be preferable toinclude a layer of material between the substrate, especially if it issoda lime glass, and the photocatalytic layer(s) to serve as a barrieragainst sodium leaching in particular If this layer is close to theindex of refraction of the substrate, such as silica on soda lime glass,it will not affect the optical properties of the system greatly andshould not be considered as circumventing the spirit of the inventionwith regards to contrasting optical properties between layers.

[0044] To expedite the evaporation of water on the mirror and preventthe freezing of thin films of water on the mirror, a heating element 122may optionally be provided on the fourth surface 114 b of reflectiveelement 100. Alternatively, one of the transparent front surface filmscould be formed of an electrically conductive material and hencefunction as a heater.

[0045] To illustrate the properties and advantages of the presentinvention, an example is provided below. The following illustrativeexample is not intended to limit the scope of the present invention butto illustrate its application and use. In this example, references aremade to the spectral properties of an electrochromic mirror constructedin accordance with the parameters specified in the example. Indiscussing colors, it is useful to refer to the CommissionInternationale de I'Eclairage's (CIE) 1976 CIELAB Chromaticity Diagram(commonly referred to as the L*a*b* chart) The technology of color isrelatively complex, but a fairly comprehensive discussion is given by FW Billmeyer and M Saltzman in Principles of Color Technology, 2ndEdition I Wiley and Sons Inc (1981), and the present disclosure, as itrelates to color technology and terminology, generally follows thatdiscussion. On the L*a*b* chart L* defines lightness, a* denotes thered/green value, and b* denotes the yellow/blue value Each of theelectrochromic media has an absorption spectra at each particularvoltage that may be converted to a three-number designation, theirL*a*b* values To calculate a set of color coordinates, such as L*a*b*values, from the spectral transmission or reflectance, two additionalitems are required. One is the spectral power distribution of the sourceor illuminant. The present disclosure uses CIE Standard Illuminant D₆₅.The second item needed is the spectral response of the observer. Thepresent disclosure uses the 2 degree CIE standard observer. Theilluminant/observer combination used is represented as D₆₅/2 degree.Many of the examples below refer to a value Y from the 1931 CIE Standardsince it corresponds more closely to the spectral reflectance than L*.The value C*, which is also described below, is equal to the square rootof (a*)²+(b*)², and hence, provides a measure for quantifying colorneutrality. To obtain an electrochromic mirror having relative colorneutrality, the C* value of the mirror should be less than 20.Preferably, the C* value is less than 15, and more preferably is lessthan about 10.

EXAMPLE

[0046] Two identical electrochromic mirrors were constructed having arear element made with 2.2 mm thick glass with a layer of chrome appliedto the front surface of the rear element and a layer of rhodium appliedon top of the layer of chrome using vacuum deposition. Both mirrorsincluded a front transparent element made of 1.1 mm thick glass, whichwas coated on its rear surface with a transparent conductive ITO coatingof {fraction (1/2)} wave optical thickness The front surfaces of thefront transparent elements were covered by a coating that included afirst layer of 200 Å thick TiO₂, a second layer of 250 Å thick SiO₂, athird layer of 1000 Å TiO₂, and a fourth layer of 500 Å thick SiO₂. Foreach mirror, an epoxy seal was laid about the perimeter of the twocoated glass substrates except for a small port used to vacuum fill thecell with electrochromic solution. The seal had a thickness of about 137microns maintained by glass spacer beads The elements were filled withan electrochromic solution including propylene carbonate containing 3percent by weight polymethylmethacrylate, 30 mM Tinuvin P (UV absorber),38 mM N,N′-dioctyl-4, 4′bipyridinium bis(tetrafluoroborate), 27 mM5,10-dihydrodimethylphenazine and the ports were then plugged with a UVcurable adhesive. Electrical contact buss clips were electricallycoupled to the transparent conductors.

[0047] In the high reflectance state (with no potential applied to thecontact buss clips), the electrochromic mirrors had the followingaveraged values: L*=78.26, a*=−2.96, b*=4.25, C*=5.18, and Y=53.7. Inthe lowest reflectance state (with a potential of 1.2 V applied), theelectrochromic mirrors had the following averaged values: L*=36.86,a*=6.59, b*=−3 51, C*=7.5, and Y=9.46. The average contact angle that adrop of water formed on the surfaces of the electrochromic mirrors afterit was cleaned was 7°

[0048] For purposes of comparison, two similar electrochromic mirrorswere constructed, but without any first surface coating. These twomirrors had identical construction. In the high reflectance state, theelectrochromic mirrors had the following averaged values: L*=78 93,a*=2.37, b*=2.55, C*=3.48, and Y=54.81 In the lowest reflectance state,the electrochromic mirrors had the following averaged values: L*=29.46,a*=0.55, b*=−16.28, C=16.29, and Y=6 02. As this comparison shows, theelectrochromic mirrors having the inventive hydrophilic coatingunexpectedly and surprisingly had better color neutrality than similarlyconstructed electrochromic mirrors not having such a hydrophilic coatingAdditionally, the comparison shows that the addition of the hydrophiliccoating does not appreciably increase the low-end spectral reflectanceof the mirrors

[0049] The present invention thus provides a hydrophilic coating thatnot only is suitable for an electrochromic device, but actually improvesthe color neutrality of the device

[0050] Although the example cited above uses a vacuum depositiontechnique to apply the coating, these coatings can also be applied byconventional sol-gel techniques. In this approach, the glass is coatedwith a metal alkoxide made from precursors such as tetra isopropyltitanate, tetra ethyl ortho silicate, or the like. These metal alkoxidescan be blended or mixed in various proportions and coated onto glassusually from an alcohol solution after being partially hydrolyzed andcondensed to increase the molecular weight by forming metal oxygen metalbonds. These coating solutions of metal alkoxides can be applied toglass substrates by a number of means such as dip coating, spin coating,or spray coating. These coatings are then fired to convert the metalalkoxide to a metal oxide typically at temperatures above 450° C. Veryuniform and durable thin film can be formed using this method. Since avacuum process is not involved, these films are relatively inexpensiveto produce. Multiple films with different compositions can be built upprior to firing by coating and drying between applications. Thisapproach can be very useful to produce inexpensive hydrophilic coatingson glass for mirrors, especially convex or aspheric mirrors that aremade from bent glass. In order to bend the glass, the glass must beheated to temperatures above 550° C. If the sol-gel coatings are appliedto the flat glass substrate before bending (typically on what will bethe convex surface of the finished mirror), the coatings will fire to adurable metal oxide during the bending process. Thus, a hydrophiliccoating can be applied to bent glass substrates for little additionalcost. Since the majority of outside mirrors used in the world today aremade from bent glass, this approach has major cost benefits It should benoted that some or all of the coatings could be applied by this sol-gelprocess with the remainder of the coating(s) applied by a vacuumprocess, such as sputtering or E-beam deposition For example, the firsthigh index layer and low index layer of, for instance, TiO₂ and SiO₂,could be applied by a sol-gel gel technique and then the top TiO₂ andSiO₂ layer applied by sputtering. This would simplify the requirementsof the coating equipment and yield cost savings. It is desirable toprevent migration of ions, such as sodium, from soda lime glasssubstrates into the photocatalytic layer. The sodium ion migration rateis temperature dependent and occurs more rapidly at high glass bendingtemperatures. A sol-gel formed silica or doped silica layer, forinstance phosphorous doped silica, is effective in reducing sodiummigration. This barrier underlayer can be applied using a sol-gelprocess. This silica layer could be applied first to the base glass orincorporated into the hydrophilic stack between the photocatalytic layerand the glass.

[0051] In general, the present invention is applicable to anyelectrochromic element including architectural windows and skylights,automobile windows, rearview mirrors, and sunroofs. With respect torearview mirrors, the present invention is primarily intended foroutside mirrors due to the increased likelihood that they will becomefoggy or covered with mist. Inside and outside rearview mirrors may beslightly different in configuration. For example, the shape of the frontglass element of an inside mirror is generally longer and narrower thanoutside mirrors. There are also some different performance standardsplaced on an inside mirror compared with outside mirrors For example, aninside mirror generally, when fully cleared, should have a reflectancevalue of about 70 percent to about 85 percent or higher, whereas theoutside mirrors often have a reflectance of about 50 percent to about 65percent Also, in the United States (as supplied by the automobilemanufacturers), the passenger-side mirror typically has a non-planarspherically bent or convex shape, whereas the driver-side mirror 111 aand inside mirror 110 presently must be flat In Europe, the driver-sidemirror 111 a is commonly flat or aspheric, whereas the passenger-sidemirror 111 b has a convex shape In Japan, both outside mirrors have anon-planar convex shape.

[0052] The fact that outside rearview mirrors are often non-planarraises additional limitations on their design. For example, thetransparent conductive layer applied to the rear surface of a non-planarfront element is typically not made of fluorine-doped tin oxide, whichis commonly used in planar mirrors, because the tin oxide coating cancomplicate the bending process and it is not commercially available onglass thinner than 2.3 mm. Thus, such bent mirrors typically utilize alayer of ITO as the front transparent conductor ITO, however, isslightly colored and adversely introduces blue coloration into thereflected image as viewed by the driver. The color introduced by an ITOlayer applied to the second surface of the element may be neutralized byutilizing an optical coating on the first surface of the electrochromicelement To illustrate this effect, a glass element coated with a halfwave thick ITO layer was constructed as was a glass element coated witha half wave thick ITO layer on one side and the hydrophilic coatingdescribed in the above example on the other side. The ITO-coated glasswithout the hydrophilic coating had the following properties: L*=37.09,a*=8.52, b*=−21.12, C*=22.82, and a first/second surface spectralreflectance of Y=9.58. By contrast, the ITO-coated glass that includedthe inventive hydrophilic coating of the above-described exampleexhibited the following properties L*=42 02, a*=2.34, b*=−8 12 C*=8.51,and a first/second surface spectral reflectance Y=12 51 As evidenced bythe significantly reduced C* value, the hydrophilic coating serves as acolor suppression coating by noticeably improving the coloration of aglass element coated with ITO Because outside rearview mirrors are oftenbent and include ITO as a transparent conductor, the ability to improvethe color of the front coated element by adding a color suppressioncoating to the opposite side of the bent glass provides manymanufacturing advantages.

[0053] Other light attenuating devices, such as scattered particledisplays (such as those discussed in aaa, bbb, ccc, patents) or liquidcrystal displays (such as those discussed ddd, eee, fff patents), canalso benefit from the application of these principles In devices wherethe light attenuating layer is between two pieces of glass or plastic,the same basic constraints and solutions to those constraints willapply. The color and reflectivity of a first surface hydrophilic layeror layer stack can impart substantial color and reflectivity to thedevice in the darkened state even when this first surface layer stackdoes not appreciably affect the bright state characteristics.Adjustments to the first surface layer stack similar to those discussedfor an electrochromic device will, therefore, affect the color and/orreflectivity of the darkened device advantageously. The same will applyto adjustments made to the second surface of the device or to the colorof the darkening layer itself.

[0054] These principles can also be applied to devices such as insulatedwindows where the light attenuating device may be contained within astructure having, for example, an additional outer and inner pane ofglass for insulation purposes. The hydrophilic coating would, in thiscase, need to be on the outside of the multilayer structure, but wouldaffect the color of the darkened device similarly. The techniquesdiscussed could be advantageously applied in this structure Thosefamiliar with the art can see that coatings could or would betransferred to a different surface or surfaces of the device because ofthe additional surfaces either available or interposing within thedevice

[0055] The above description is considered that of the preferredembodiments only Modifications of the invention will occur to thoseskilled in the art and to those who make or use the invention.Therefore, it is understood that the embodiments shown in the drawingsand described above are merely for illustrative purposes and notintended to limit the scope of the invention, which is defined by thefollowing claims as interpreted according to the principles of patentlaw, including the Doctrine of Equivalents.

The invention claimed is:
 1. A rearview mirror for a vehicle comprising:an electrochromic mirror element having a reflectivity that may hevaried in response to an applied voltage so as to exhibit at least ahigh reflectance state and a low reflectance state, and a hydrophilicoptical coating applied to a front surface of said electrochromic mirrorelement, wherein said rearview mirror exhibits a spectral reflectance ofless than 20 percent in said low reflectance state.
 2. The rearviewmirror as defined in claim 1, wherein said hydrophilic optical coatingis sufficiently hydrophilic such that water droplets on a front surfaceof said optical coating exhibit a contact angle of less than about 30°.3. The rearview mirror as defined in claim 1, wherein said hydrophilicoptical coating is sufficiently hydrophilic such that water droplets ona front surface of said optical coating exhibit a contact angle of lessthan about 20°.
 4. The rearview mirror as defined in claim 1, whereinsaid hydrophilic optical coating is sufficiently hydrophilic such thatwater droplets on a front surface of said optical coating exhibit acontact angle of less than about 10°.
 5. The rearview mirror as definedin claim 1, wherein said rearview mirror exhibits a C* value less thanabout 20 in both said high and low reflectance stales
 6. The rearviewmirror as defined in claim 1, wherein said hydrophilic optical coatingincludes an outermost hydrophilic enhancement layer having a thicknessof less than about 800 Å.
 7. The rearview mirror as defined in claim 6,wherein said hydrophilic enhancement layer is made of silicon dioxide.8. The rearview mirror as defined in claim 6, wherein said hydrophilicoptical coating includes a photocatalytic layer immediately underlyingsaid outermost hydrophilic layer.
 9. The rearview mirror as defined inclaim 8, wherein said photocatalytic layer is made of primarily titaniumdioxide.
 10. The rearview mirror as defined in claim 9, wherein saidhydrophilic layer is made of primarily silicon dioxide. 11 The rearviewmirror as defined in claim 1, wherein said hydrophilic optical coatingincludes an outermost hydrophilic enhancement layer having a thicknessof less than about 500 Å.
 12. The rearview mirror as defined in claim 1,wherein said hydrophilic optical coating includes an outermost layer ofsilicon dioxide and a titanium dioxide layer disposed immediately behindsaid outermost layer
 13. The rearview mirror as defined in claim 1,wherein said hydrophilic optical coating includes a first layer having ahigh refractive index disposed on a front surface of reflective element,a second layer having a low refractive index disposed on said firstlayer, a third layer of titanium dioxide disposed on said second layer;and a fourth layer of silicon dioxide disposed on said third layer. 14.The rearview mirror as defined in claim 13, wherein said first layerincludes titanium dioxide.
 15. The rearview mirror as defined in claim14, wherein said second layer includes silicon dioxide.
 16. The rearviewmirror as defined in claim 13, wherein said second layer includessilicon dioxide. 17 The rearview mirror as defined in claim 14, whereinsaid fourth layer has a thickness of less than about 800 Å. 18 Therearview mirror as defined in claim 14, wherein said fourth layer has athickness of less than about 500 Å.
 19. The rearview mirror as definedin claim 1, wherein said rearview mirror exhibits a spectral reflectanceof less than 15 percent in said low reflectance state.
 20. The rearviewmirror as defined in claim 1, wherein said hydrophilic optical coatingis sufficiently hydrophilic such that water droplets on a front surfaceof said optical coating exhibit a contact angle of less than about 20°,and said rearview mirror exhibits a C* value less than about 20 in bothsaid high and low reflectance states.
 21. The rearview mirror as definedin claim 20, wherein said rearview mirror exhibits a spectralreflectance of less than 15 percent in said low reflectance state 22.The rearview mirror as defined in claim 21, wherein said hydrophilicoptical coating includes an outermost hydrophilic layer having athickness of less than about 800 Å. 23 The rearview mirror as defined inclaim 21, wherein said hydrophilic optical coating includes an outermosthydrophilic layer having a thickness of less than about 500 Å 24 Arearview mirror for a vehicle comprising: an electrochromic mirrorelement having a reflectivity that may he varied in response to anapplied voltage so as to exhibit at least a high reflectance state and alow reflectance state, and a hydrophilic optical coating applied to afront surface of said electrochromic mirror element, wherein saidrearview mirror exhibits a C* value less than about 20 in both said highand low reflectance states.
 25. The rearview mirror as defined in claim24, wherein said hydrophilic optical coating includes an outermosthydrophilic layer having a thickness of less than about 800 Å.
 26. Therearview mirror as defined in claim 24, wherein said hydrophilic opticalcoating includes an outermost hydrophilic layer having a thickness ofless than about 500 Å.
 27. The rearview mirror as defined in claim 24,wherein said hydrophilic optical coating is sufficiently hydrophilicsuch that water droplets on a front surface of said hydrophilic opticalcoating exhibit a contact angle of less than about 20°.
 28. Anelectro-optic device comprising: an electro-optic element having avariable transmittance, said electro-optic element having a frontsurface and a rear surface; and a hydrophilic optical coating comprisingin sequence. a first layer having a high refractive index. a secondlayer having a low refractive index, a third layer of titanium dioxide,and a fourth layer of silicon dioxide provided as an outermost layer 29.The electro-optic device as defined in claim 28, wherein said firstlayer of said hydrophilic optical coating includes titanium dioxide. 30.The electro-optic device as defined in claim 29, wherein said secondlayer of said hydrophilic optical coating includes silicon dioxide. 31.The electro-optic device as defined in claim 28, wherein said secondlayer of said hydrophilic optical coating includes silicon dioxide. 32.The electro-optic device as defined in claim 28, wherein said electrooptic element is an electrochromic element including a reflectivesurface so as to function as an electrochromic mirror
 33. Theelectro-optic device as defined in claim 32, wherein said electrochromicelement includes an electrochromic medium having a color that it is lessabsorbing of green light than other colors of light when theelectrochromic element is activated. 34 The electro-optic device asdefined in claim 28, wherein said electro-optic device exhibits a C*value of less than about 20
 35. The electro-optic device as defined inclaim 28, wherein said electro-optic device has a C* value less thanabout
 15. 36. The electro-optic device as defined in claim 28, whereinsaid electro-optic device has a C* value less than about
 10. 37. Anelectro-optic device comprising: an electro-optic element having avariable transmittance, said electro-optic element having a frontsurface and a rear surface; and a self-cleaning hydrophilic opticalcoating disposed on one of the front and rear surfaces of saidelectro-optic element, said hydrophilic optical coating comprisingalternating layers of a photocatalytic material having a high index ofrefraction and a hydrophilic material having a low refractive index. 38.The electro-optic device as defined in claim 37, wherein saidphotocatalytic material is titanium dioxide. 39 The electro-optic deviceas defined in claim 38, wherein said hydrophilic material is silicondioxide
 40. The electro optic device as defined in claim 37, whereinsaid hydrophilic material is silicon dioxide.
 41. The electro-opticdevice as defined in claim 37, wherein a first layer of said coatingadjacent the finest surface of the electro-optic element is made of saidphotocatalytic material
 42. The electro-optic device as defined in claim37, wherein said electro-optic element includes: front and rearsubstrates sealably bonded together to form a chamber, each substrateincluding a front and a rear surface, the front surface of said frontsubstrate serving as the front surface of said electrochromic element;an electrochromic medium contained in said chamber; a transparent firstelectrode carried on one of the front surface of said rear substrate andsaid rear surface of said front substrate; and a second electrodecarried on the front surface of said rear substrate
 43. Theelectro-optic device as defined in claim 42, wherein said electro-opticelement further includes a reflector carried on the rear surface of saidrear substrate. 44 The electro-optic device as defined in claim 42wherein aid second electrode is reflective 45 The electro optic deviceas defined in claim 42, wherein said electrochromic medium is asolution-phase electrochromic medium.
 46. An external electrochromicrearview mirror for a vehicle comprising: front and rear elements eachhaving front and rear surfaces and being sealably bonded in spaced-apartrelation to form a chamber; a transparent first electrode carried on oneof said front surface of said rear element and said rear surface of saidfront element; a second electrode carried on one of said front surfaceof said rear element and said rear surface of said front element,wherein either said second electrode is reflective or a separatereflector is provided on a surface of said rear element; anelectrochromic medium contained in said chamber in electrical contactwith said first and second electrodes; a first layer having a highrefractive index disposed in front of said front surface of said frontelement; a second layer having a low refractive index disposed on saidfirst layer; a third layer of a photocatalytic material disposed on saidsecond layer, and a fourth layer of a hydrophilic material disposed onsaid third layer
 47. The external electrochromic rearview mirror asdefined in claim 46, wherein said third layer includes titanium dioxide.48. The external electrochromic rearview mirror as defined in claim 47,wherein said fourth layer includes silicon dioxide
 49. The externalelectrochromic rearview mirror as defined in claim 46, wherein saidfourth layer includes silicon dioxide
 50. The external electrochromicrearview mirror as defined in claim 46, wherein said first layerincludes titanium dioxide.
 51. The external electrochromic rearviewmirror as defined in claim 50, wherein said second layer includessilicon dioxide.
 52. The external electrochromic rearview mirror asdefined in claim 46, wherein said second layer includes silicon dioxide53. The external electrochromic rearview mirror as defined in claim 46,wherein said fourth layer has a thickness of less than about 800 Å. 54.The external electrochromic rearview mirror as defined in claim 46,wherein said rearview mirror is non-planar. 55 A non-planar rearviewmirror for a vehicle comprising: a non-planar mirror element includingat least one bent glass element; and a hydrophilic sol-gel coating onsaid bent glass element, wherein said hydrophilic sol-gel coating isapplied just prior to bending of the glass element and is fired in theprocess of bending the glass element.
 56. A method of making anon-planar rearview mirror for a vehicle comprising the steps of:applying a hydrophilic sol-gel coating on a glass element; and heatingand bending the coated glass element.
 57. A non-planar rearview mirrormade using the method defined in claim
 56. 58. An electrochromic devicecomprising an electrochromic element having a transmittance that may bevaried in response to an applied voltage, said electrochromic elementincluding a glass substrate having a rear surface coated in atransparent conductive material, and a front surface having a colorsuppression coating formed thereon for suppressing color imparted to thedevice by the transparent conductive material.
 59. The electrochromicdevice as defined in claim 58, wherein said electrochromic elementfurther comprises a reflective layer so as to function as a mirrorhaving variable reflectivity. 60 The electrochromic device as defined inclaim 58, wherein said color suppression coating is hydrophilic.
 61. Anelectrochromic device comprising an electrochromic element having atransmittance that may be varied in response to an applied voltage, saidelectrochromic element including a glass substrate having a rear surfacecoated in a transparent conductive material, and a front surface havinga hydrophilic coating formed thereon, wherein the thickness of saidtransparent conductive coating is selected for suppressing colorimparted to the device by said hydrophilic coating.